A private telephone system may be provided in any of three basic configurations, namely, a private branch exchange, a key telephone system or a special line group terminated at a central office (CO) exchange which includes a centrex feature. A centrex feature, hereinafter referred to as centrex, provides private branch exchange (PBX) functionality for the special line group. PBX functionality is characterized by a simplified number plan which identifies extensions by two, three, or four digit numbers, depending upon the size of the PBX. This is in contrast to a minimum of seven digits typically required for a directory number (DN) serviced by a public CO exchange. In the event that a PBX user or a centrex user desires to originate a telephone call to a subscriber of the public telephone service, such is indicated to the PBX or to the centrex by a predetermined digit, a nine for example, which is dialled before the digits of the subscriber's DN are dialled.
Key telephone systems tend to be characterized by a plurality of key telephone sets being connected to a plurality of CO telephone lines and a separate set of leads, exclusive to the key telephone system which may be utilized for intercommunications between the key telephone sets without recourse to accessing the CO. This is referred to as an intercom feature and may be voice or dial activated. In any event, before dialling, the key telephone system use must decide which means of communication will be used. Thereafter, the user depresses an appropriate key on the telephone set to select an intercom line or a CO line.
Key telephone systems are typified as being of low cost at small sizes, as compared to PBX in which common equipment is a major expense. For example, three CO lines may be shared among six to twelve telephone sets, each of which has access to any of the lines for exercising basic well known features such as "hold", "privacy" and "intercom", in addition to plain ordinary telephone service (POTS).
As before mentioned, the basic features available via the PBX are also available at the central office exchange, via centrex. These may include a wealth of calling features such as abbreviated dialling, speed calling, dial-by-name, ring again, conference, to name but a few.
Many of these features are thus available via the key telephone system, specifically for those of the key telephone sets that are served by one or more PBX or centrex lines. Of course the key telephone system user must be aware of each of the features and how to access same, before any user advantage is experienced. In the centrex example, if the key telephone system user wishes to conference with several numbers, one of them being outside of the key telephone systems, the key telephone system user must remember to access the conference feature in the CO, while ignoring the key telephone system conference feature. This is merely a simple example of the knowledge required of the user in order that the advantages of telephony features may be made of use. It has been found that as more features become available, access requirements in the mix of locally and centrally provided features has become confusing to many telephone users. Consequently, many potential feature users simply restrict themselves to using POTS, as methods of feature utilization are incomprehensible for them.